As to electrochemical devices typically represented by electric double layer capacitors, improvement has been demanded for the performance such as higher withstanding voltage, longer life, and higher heat resistance. Therefore, it is required that the electrolyte solution used therefore is electrochemically stabler. As a conventional electrolyte solution for electric double layer capacitors, electrolyte solutions using cyclic carbonates or cyclic lactones with high electrochemical stability have been used mainly. Such electrolyte solutions are disclosed, for example, in Japanese Patent Unexamined Publication No. H07-283083. As a typical cyclic carbonate, propylene carbonate has been used generally.
However, for making the electric double layer capacitor have higher withstanding voltage, longer life, and higher heat resistance, propylene carbonate is not sufficient. Propylene carbonate has a structural portion in which two or more oxygen atoms are bonded directly to one carbon atom. Accordingly, a gas mainly of carbon dioxide (CO2) is generated in a great amount along with decomposition, and this causes deterioration of the characteristics of the electrochemical device.
That is, an electrolyte solution constituted with a solvent of higher electrochemical stability than that of propylene carbonate is required. The higher electrochemical stability means herein that an electrochemical device using the same can be utilized at a higher voltage for a longer period of time while suppressing the change of characteristics thereof such as capacitance and internal resistance within a predetermined range.